The present Applicant has developed a plethora of high-speed inkjet printers employing stationary Memjet® printheads which extend across a media width and print in a single pass of the print media. By contrast, most inkjet printers utilize a scanning printhead, which traverses across the media width printing in swathes.
High-speed pagewidth printing necessarily places additional demands on the design of the printhead compared to traditional types of inkjet printhead. The nozzle devices must have a self-cooling design, high chamber refill rates and high thermal efficiency. To this end, the Applicant has developed a range of thermal bubble-forming printheads, including those with suspended resistive heater elements (as described in, for example, U.S. Pat. Nos. 6,755,509; 7,246,886; 7,401,910; and 7,658,977, the contents of which are incorporated herein by reference) and those with embedded (“bonded”) resistive heater elements (as described in, for example, U.S. Pat. Nos. 7,377,623; 7,431,431; US 2006/250453; and U.S. Pat. No. 7,491,911, the contents of which are incorporated herein by reference).
Nozzle devices having uncoated suspended heater elements offer the advantages of efficient heat transfer from the heater element to the ink and self-cooling characteristics, resulting in high print speeds. However, uncoated suspended heater elements are typically less robust than their bonded counterparts.
One approach to improving printhead lifetime is to coat the heater elements with a layer of protective coating. For example, U.S. Pat. No. 6,719,406 (assigned to the present Applicant) describes suspended heater elements having a conformal protective coating, which improves the robustness of the heater element and improves printhead lifetime. However, protective coatings are undesirable for a number of reasons—they reduce the efficiency of heat transfer from the resistive heater elements to the surrounding ink; they consequently affect the self-cooling characteristics; and they introduce additional MEMS fabrication challenges.
Therefore, it is generally preferable to employ uncoated (“naked”) heater elements in Memjet® printheads. To some extent, the choice of heater material can mitigate the effects of using uncoated heater elements, which are directly exposed to the ink. For example, U.S. Pat. No. 7,431,431 describes the use of a self-passivating titanium aluminium nitride heater element, which has improved lifetime compared to more conventional materials used in the art. Nevertheless, there is still a need to improve the lifetimes of Memjet® printheads employing uncoated heater elements, which are susceptible to failure via both kogative and corrosive mechanisms.
Aqueous dye-based inks are considered to be well-suited for use in high-speed printing, because they usually exhibit minimal kogation compared to pigment-based inks. However, dye-based inks typically have the disadvantages of poorer lightfastness, poorer optical densities and poorer fixability (“rub-fastness”) compared to pigment-based inks. The optical density of black ink is particularly important, because consumers generally prefer printed text to be a rich, crisp black color without any hints of grayness.
EP-A-0947567 describes an aqueous ink formulation comprising a self-dispersible carbon black pigment and a resin encapsulating a coloring material.
JP 2001-288390 describes the use of two different black inks having different surface tensions for reducing drying times.
U.S. Pat. No. 5,976,233 describes an aqueous inkjet ink comprising a self-dispersible pigment, trimethylolpropane, diethyleneglycol and glycerol.
It would desirable to provide a method of printing pigment-based inks from a high-speed printhead, such as the Applicant's Memjet® printhead. It would further be desirable to print pigment-based inks whilst optimizing optical density, fixability and/or printhead lifetime.